Despite their superhero nicknames the adorable puppies are actually much smaller and weaker then the rest of the litter and one of them, a female, died shortly after being born.

The male pup is still alive but struggling and is starting to lose his unusual colouring.

Vet Daniel Valverde is probing the case, and said: "There are no other recorded cases of this happening in Spanish scientific literature. I am carrying out virological and bacteriological tests to try and find an answer."

SUPERHERO: The puppies have been likened to Marvel hero The Incredible Hulk [CEN]

The two Spanish puppies are not the first reported examples of green pups.

In 2012 a green Labrador puppy was born in England that lost its green hue after ten days.

The cause for the colouring at that time was given as exposure to a substance called biliverdin, which can be found in the placenta.